(1) Field of the Invention
This invention relates to a packet sending apparatus and a packet transmission system and, more particularly, to a packet sending apparatus for sending packets by priority control and a packet transmission system for transmitting packets by priority control between a sending apparatus and a receiving apparatus between which clock frequency deviation exists.
(2) Description of the Related Art
In recent years communication services provided on the basis of Ethernet (registered trademark) or the like by using internet protocol (IP) packets have developed remarkably. One of main communication services employs priority control. With the priority control, high priority packets are sent in preference to non-priority packets. That is to say, services in which communication quality discrimination is exercised are provided.
Traffic which always passes through, for example, corporate networks includes various pieces of information, such as mails sent and files transferred. Accordingly, if other pieces of information, such as voice packets sent by IP telephony, are added to this traffic, voice may be hard to listen to at the other end. In such cases, the priority control should be used. By doing so, voice packets are placed into a top priority class and are sent to the other end in preference to the other information packets. As a result, deterioration in voice quality can be prevented.
When a packet sending apparatus having a priority packet sending function receives packets, the packet sending apparatus determines the priorities of the packets and places and stores the packets in internal queues or the like according to their priorities. The packet sending apparatus then sends the packets stored in the internal queues in descending order of priority of queue.
Conventionally, a technique for making intervals at which priority packets are sent shorter than the average sending interval and for sending non-priority packets by using the remaining time given is proposed as a technique for transmitting packets (see, for example, WO 2003/017577 (paragraphs [0117] to [0119] and FIG. 1).
A packet sending apparatus and a packet receiving apparatus each have their clock sources. Signal provisions which comply with the IEEE802.3 standard permit packet transmission between a packet sending apparatus and a packet receiving apparatus at (frequency of each clock source) ±100 p.p.m (p.p.m= 1/1,000,000). Accordingly, in packet transmission between a packet sending apparatus and a packet receiving apparatus, packet information may be lost due to clock frequency deviation between the packet sending apparatus and the packet receiving apparatus (if the speed of a clock source of the packet sending apparatus is high and the speed of a clock source of the packet receiving apparatus is low). Usually loss of packet information due to clock frequency deviation is permitted by the standard and is originally hopeless. If priority control is exercised in such a sending-receiving system, information included in a priority packet may be lost.
In this case, it is possible to prevent loss of packet information by merely lowering the output rate of the packet sending apparatus. However, this method lowers a transmission rate at a fixed rate. Usually a receiving buffer is located on the packet receiving apparatus side. As a result, the discard of packet information corresponding to clock frequency deviation can be prevented for a certain period of time. Furthermore, the receiving buffer can absorb variation in the amount of data transmitted (the packet sending apparatus does not always perform transmission at the highest transmission rate (full wire rate)). If the transmission rate of the packet sending apparatus is lowered with the clock frequency deviation taken into consideration, the efficiency of transmission in the entire system decreases.
Loss of packet information caused by the clock frequency deviation occurs randomly. This means that regardless of whether a packet sent by the packet sending apparatus is a priority packet or a non-priority packet (packet in, for example, best effort service for which a band is not guaranteed), information included in the packet is lost. If such a situation occurs, the packet sending apparatus which exercises priority control becomes meaningless.